When machining precise surfaces of a workpiece, it is often necessary to measure them using a precise measuring instrument, such as a scanning profilometer. In these situations, it is necessary to register the measurements of the profilometer with the motion coordinates of the machine tool. A fixture is used to hold the workpiece while allowing the workpiece to be removed and replaced in the machine tool and profilometer in a repeatable fashion. For example, mounting the workpiece in the fixture allows it to be installed in the machine tool, removed and re-installed, and the position of the workpiece relative to the machine tool coordinate system will be identical to the original position.
Consider a turning machine, for example, which has a controllable spindle orientation and controllable tool motion axes, such that it is possible to control the machine to move the tool to a specific location on the workpiece. Directing the machine to address a specific measured point on the workpiece is problematic because it is necessary to identify both the relative position and orientation of the measurement data with respect to the machine tool coordinate system. Accordingly, it will be appreciated that it would be highly desirable to have a method for machine tool and profilometer registration so that measured surface data can be registered with commanded machining actions in all three degrees of freedom required to define surface points.
European patent document EP 02 57 164 A1 discloses a device for relative positioning between a tool and a workpiece, and German patent document DE 40 38 849 A1 discloses an electronic measurement head. Both make use of electric current in the positioning device. Electric currents can be erratic due to resistance changes in the electric current path, such as can occur between different workpieces or the same workpiece at different times.
Thus, there remains a need for a method for machine tool and profilometer registration that allows accurate measurement of different workpieces or the same workpiece at different times.